A transmit continuous-time filter (TX-CTF) is a frequency-selective circuit that is typically included in the transmitter portion of some types of cellular telephones (also referred to as handsets). A TX-CTF typically receives the output of a digital-to-analog converter (DAC) and attenuates the DAC aliasing and noise. The output of the TX-CTF is typically provided to an active upconversion mixer that upconverts the signal from a baseband frequency to the desired radio frequency (RF) band for transmission.
Some tri-mode cellular handsets support Wideband Code Division Multiple Access (WCDMA) modulation, the Gaussian Minimum Shift Keying (GMSK) modulation used in the Global System for Mobile telecommunication (GSM) standard, and the 8-Phase Shift Keying (8PSK) modulation used in the Enhanced Data Rates for Global Evolution (EDGE) standard (also known as the Enhanced Data Rates for GSM Evolution standard). Enabling all three of the above modes in the same handset imposes stringent performance requirements on the TX-CTF, including high current drive capability, high linearity, low input referred noise, and low bandpass ripple. Designing a TX-CTF that meets all of these requirements can be problematic. Straightforward solutions to these challenges that may be proposed, such as increasing current, can introduce other problems. For example, high TX-CTF current consumption can lead to an undesirably high current drain on the cellular handset battery. Minimizing battery current drain is desirable so that talk time and standby time (i.e., the amount of time the handset can be used before the battery requires recharging) can be maximized and battery size can be minimized.
A typical TX-CTF 10 is shown in FIG. 1. As a typical cellular handset transmitter uses a form of quadrature modulation, TX-CTF 10 includes an in-phase (I) portion 12 and a quadrature (Q) portion 14. As portions 12 and 14 are essentially identical, only portion 12 is described in detail herein. Portion 12 includes two sections 16 and 18. Section 16 can be, for example, a 2nd-order biquadratic stage, and section 18 can be, for example, a 1st-order real pole stage. Section 16 includes a first amplifier 20 as well as passive circuitry that can include, for example, capacitors 22, 24 and 26, and resistors 28, 30, 32, 34, 36 and 38. The passive circuitry can be selected and connected to first amplifier 20 in an arrangement that defines the desired filter parameters, such as the filter poles and/or zeroes that characterize a 2nd-order biquadratic filter. Similarly, section 18 includes a second amplifier 40 as well as passive circuitry that can include, for example, capacitors 42 and 44, and resistors 46, 48, 50 and 52. The passive circuitry of section 18 likewise can be selected and connected to second amplifier 40 in an arrangement that defines the desired filter parameters, such as the filter poles and/or zeroes that characterize a 1st-order real pole filter.
In operation, a differential input signal V1 (the negative side of which is represented in FIG. 1 as “V1_N,” and the positive side as “V1_P”) is provided to stage 16, which outputs a signal V2 (the negative side of which is represented as “V2_N,” and the positive side as “V2_P”). The signal V2 is in turn provided to stage 18, which outputs a signal V3 (the negative side of which is represented as “V3_N” and the positive as “V3_P”).
As in a typical cellular handset, the in-phase (I) and quadrature (Q) outputs of TX-CTF 10 are provided to active upconversion mixers 54 and 56, respectively. Each of upconversion mixers 54 and 56 is typically of the Gilbert cell type, which presents a high-impedance load to TX-CTF 10. The TX-CTF 10 is readily capable of driving the high impedance load with low current and maintaining the required linearity.
It would be desirable to provide a TX-CTF for a multi-mode cellular handset that can meet the above-described performance criteria or similar performance criteria without consuming excessive current.